Conventionally, display devices having a liquid crystal panel as a display panel have included backlight devices that illuminate the display panel from the rear (refer to Patent Document 1, for example). FIG. 13 is a schematic perspective view showing a configuration example of a conventional backlight device 100. FIGS. 14A and 14B are schematic cross-sectional views of the backlight device shown in FIG. 13, FIG. 14A being a cross-sectional view along the line Z1-Z1, FIG. 14B being a cross-sectional view along the line Z2-Z2. The backlight device 100 shown in FIGS. 13, 14A and 14B is a so-called edge-lit (side-lit) backlight device.
As shown in FIGS. 13, 14A, and 14B, the conventional backlight device 100 includes a light source 101, a light guide plate 102, a plurality of sheets 103, a reflective sheet 104, a resin frame 105, a light-shielding tape 106, and a lamp cover 107. The light emitted by the light source 101 enters the light guide plate 102 from a side face thereof, and is emitted from the upper surface of the light guide plate 102 in a planar form. The plurality of sheets 103 include a diffusion sheet and a lens sheet, for example, and in this example, a diffusion sheet 103a is disposed at the very top. The diffusion sheet 103a evenly diffuses light emitted from the light guide plate 102 towards the display surface side.
In the conventional backlight device 100, the light-shielding tape 106 is bonded to the upper surface of the peripheral edge of the diffusion sheet 103a and the resin frame 105 so as to cover these, and thus, the diffusion sheet 103a and the resin frame 105 are fixed in place. According to this configuration, it is possible to prevent the occurrence of bright lines in the peripheral edge of the display region of the liquid crystal display element (not shown) on the upper side of the backlight device 100, and it is possible to prevent the sheets 103 from shifting in position.
However, the conventional backlight device 100 described above has the following problems. The sheets (optical sheets) 103 disposed over the light guide plate 102 expand due to temperature changes. Thus, if the peripheral edge of the optical sheets 103 is covered in a frame shape by the light-shielding tape 106, the optical sheets 103 are susceptible to warping due to temperature change. Display devices with a large display size are particularly susceptible to the negative effect of warping in the optical sheet.